This invention relates to an internal combustion (IC) engine, and more particularly to an IC engine particularly suited for use in hand-held (portable) tools.
Relatively small size IC engines are well known and are commonly used to power tools such as chain saws, blowers, line trimmers, etc. Since such tools are normally carried and used by a single person, the engine must be light weight and capable of operation in different orientations (sideways or straight up, for example).
At the present time, most engines for this purpose are two-stroke air-cooled engines because they have a good power vs. weight and size ratios, do not have a complex construction, and they are all position or orientation engines. The latter feature is made possible because such engines utilize a diaphragm-type carburetor and engine lubrication is accomplished by adding lubrication oil to the fuel (typically a 40:1 fuel-to-oil mixture).
While two-stroke engines of this type work well, they have certain drawbacks. The fuel consumption rate is relatively high and the operating noise level is also high. A very important disadvantage is that the emissions levels of such engines are quite high because the exhaust includes a sizable amount of fresh fuel. The State of California regulations soon to become effective limit the amounts of hydrocarbons and carbon monoxide that may be produced, and most or all two-stroke engines presently in use will not be able to meet the California standards. It is expected that those standards will also be adopted by other states and countries.
Four-stroke IC engines are, of course, also well known and they generally have lower hydrocarbon and carbon monoxide emissions than two-stroke engines. This is true because four-stroke engines exchange the exhaust and fresh fuel/air mixture in a more positive manner with the use of valves. Four-stroke engines also in general have lower noise levels.
Relatively small four-stroke engines are available and have been used in, for example, model or hobby aircraft. While such engines are sufficiently small to be used in portable tools, they would not be satisfactory because they have a relatively complex and light duty construction. Four-stroke engines normally have an oil sump in a crankcase at the bottom of the engine and an oil pump for moving the oil to the moving parts such as the overhead valves and the valve actuating mechanisms. This type of lubricating system is not satisfactory for all-position use.
In addition, the Y. Imagawa et al. U.S. Pat. Nos. 5,176,116, dated Jan. 5, 1993, and U.S. Pat. No. 5,213,074, dated May 25, 1993 describe a lubrication system for a portable four-stroke engine, wherein some of the engine parts are lubricated by oil in a crankcase and other parts by grease which is packed around moving parts. It is questionable whether grease will provide satisfactory lubrication for engine parts that become very hot during use. In any event it is doubtful that grease is satisfactory for long-term use in an engine in field and garden use because the grease should be periodically cleaned out and repacked. This is not practical in engines used, for example, in home gardening tools.
The R. G. Everts U.S. Pat. No. 5,241,932 dated Sep. 7, 1993, and the Y. Imagawa et al. U.S. Pat. No. 5,267,536 dated Dec. 7, 1993, describe engines generally similar to the engine described in U.S. Pat. No. 5,213,074. Both patents describe lubrication systems for a four-stroke engine, including oil in a crankcase sump and a splasher. U.S. Pat. No. 5,241,932 also states that "mist ladened air" moves from the crankcase through the valve guide tubes to the valve cover, to lubricate the valves and the rocker arms.
The D. E. Stinebaugh U.S. Pat. No. 4,708,107, dated Nov. 24, 1987, describes a four-stroke engine including a crankcase-compression arrangement. A carburetor supplies a "combustible working fluid such as an air-gasoline mixture" to the crankcase, and the mixture is pumped through a "boost plenum or reservoir", through a throttle valve, to the cylinder intake valve. A cam shaft and cam followers for the intake and exhaust valves are mounted in the boost plenum.
It is therefore a general object of the present invention to provide an improved four-stroke engine.